Many methods and devices are known in the prior art for producing drinking water from contaminated water or seawater.
Such prior art devices typically include a form of pre-filter, such as an ultrafiltration (UF) or micro-filtration (MF) unit or particulate matter pre-filter, in conjunction with a reverse osmosis filter. The pre-filter serves to remove particulate matter, such as organic and/or insoluble particulate matter and thus protects the reverse osmosis filter from destruction. The reverse osmosis filter acts to remove ionic components, such as dissolved salts from seawater.
One of the inherent problems in using UF or MF membrane filters to pre-filter water prior to reverse osmosis treatment is that such filters can become clogged with particulate matter. This is especially the case in those systems where the filtrate is extremely impure and/or in large volumes, which is particularly the case where seawater is being filtered. It therefore becomes necessary to clean such filters periodically and the simplest method of doing this is by backwashing the filters. Backwashing involves reversing the flow across the filter to force and solid or particulate matter which has become entrapped in the filter cavities back into suspension. Normally, a portion of the reverse osmosis feedwater, which as already passed through the UF or MF membrane, is used to backwash the membrane filter.
Usually only 50-90% of the reverse osmosis feed actually passes through the reverse osmosis membrane to become desalinated reverse osmosis product, i.e. potable water, so losses to backwashing are acceptable.
It is also known in the prior art to use the residual reverse osmosis feed that does not pass through the membrane i.e. the reverse osmosis reject or reverse osmosis concentrate as the backwash source for the membrane backwash. This is disclosed, for example in U.S. Pat. No. 6,120,688 where a portion of the reverse osmosis feed is redirected into a CIP (Cleaning In Place) tank which can from time to time be used to provide a flow to a micro-filtration module in a reverse direction to backwash the membranes.
However, there is an inherent problem with such an approach, namely that the reverse osmosis concentrate can form scales or particles due to the concentration effects of the reverse osmosis process. Alternatively, it is possible that there is biological growth on the reverse osmosis membrane surface which can likewise contaminate the reverse osmosis concentrate. Such particles or biological material could in fact foul the clean or filtrate side of the micro-filtration or ultrafiltration membranes of residual reverse osmosis feed is used directly to backwash the micro-filtration or ultrafiltration membranes.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.